New PV Standards Published

New PV Standards Published

James Amano

Director, SEMI International Standards

Initially formed in 2007, the PV Standards Committee has rapidly become one of the most active committees in the SEMI Standards Program, and has now developed and published 15 Standards (the newest three Standards will be available in February 2011). The PV industry is increasingly recognizing the value of the SEMI Standards Program, and these new Standards, combined with other SEMI Standards applicable to PV manufacturing, will contribute towards establishing PV as an economically competitive energy source for the world.

In addition, SEMI AUX019-0211: Research Report for an Interlaboratory Study of SEMI PV1 (Test Method for Measuring Trace Elements in Silicon Feedstock for Silicon Solar Cells by High-Mass Resolution Glow Discharge Mass Spectrometry (GDMS) has been approved for publication and will soon be available on the SEMI Standards Web site.

This article will focus on SEMI PV13-0211: Test Method for Contactless Excess-Charge-Carrier Recombination Lifetime Measurement in Silicon Wafers, Ingots, and Bricks Using an Eddy-Current Sensor. The International Analytical and Test Methods Task Force began work on this standard in 2009, and the ballot was refined over three ballot cycles before its recent approval.

The Eddy-current excess-carrier-recombination lifetime standard (PV13-0211) is relevant to both the silicon semiconductor industry and especially to the PV industry. The scope of the standard includes measurements on as-grown ingots and bricks of silicon for process control of feedstock material and growth quality. It also will be used for process control for silicon device fabrication of every process step up to metallization.

Excess-carrier lifetime is a critical control parameter for the design and fabrication of silicon solar cells. While predictive lifetime-measurement techniques have long been available to PV R&D laboratories, industrial solar cell production often still relies on methods borrowed from the field of integrated circuits. With PV13-0211, SEMI codifies lifetime-measurement techniques that have been developed and widely used in photovoltaics by leading institutes and manufacturers in the field. This will facilitate the use of state-of-the-art R&D measurements for widespread industrial use. Standards based on these techniques will accelerate advances in silicon photovoltaic production technology as companies invest in robust and traceable methods of continuous improvement.

The main challenge for silicon photovoltaics is to reduce cost in order to enable widespread adoption of PV into the general electricity mix. PV13-0211 will have major effects through optimization of the production chain, from silicon feedstock to growth and solar cell fabrication. Carrier lifetime measurements provide one of the most sensitive metrics for evaluating feedstock quality, optimizing material growth, and wafer processing, including the individual steps of cleaning, etching, dopant diffusion, surface passivation and antireflection coating. Monitoring each of these processes will enable optimization both in the direction of higher-efficiency designs and processes, as well as allow data-based evaluations of less-expensive processes and materials. Photovoltaic production is still in a nascent stage, and process control at every step is critical to making the decisions required for scaling up this industry to the levels necessary to meet the demands for clean electricity.

PV13-0211, for the measurement of a parameter central to solar cell performance, will bolster technological growth in PV by facilitating the transactions between companies through the use of qualifying silicon ingots, bricks and wafers at the point of sale, as well as qualifying wafer-process equipment for use in the production lines. During solar cell fabrication, the use of carrier-lifetime metrics improves the yield and efficiencies of solar cells. The more widespread introduction of a common platform for carrier lifetime measurement in R&D laboratories and industry will facilitate the rapid introduction of new designs and processes into the industry. Although nominally aimed at silicon PV, the standard is also a major advance in the standards portfolio useful to the semiconductor industry.

The primary author of PV13-0211, Ron Sinton, of Sinton Instruments, speaks about the importance of standards: “Industry is influenced by standards in their equipment purchase decisions. Therefore, to influence the direction of silicon PV outside of the R&D labs, it is essential to be involved in the standards process. To ignore it is to risk being marginalized as more and more of the R&D and innovation in silicon PV is done within industry. To sell test and measurement equipment to industry, it is quite useful to have a standard that discusses the main issues with using the technique.”

He continues that “on a more idealistic level, the use of different tool sets in R&D laboratories and in industry impedes the flow of innovation. The quick implementation of new ideas from institutes and universities into production lines requires that the industry and R&D maintain a common foundation. This facilitates the evolution from demonstrations on small sets of wafers to validation with large sample sets in the production environment. The use of statistics available from large trials is a valuable tool in solar cell optimization. This information is greatly enhanced if there is process control data taken with the same state-of-the-art techniques as has been used by the university and institute counterparts in prototyping new ideas.”